I have literally the exact same car as you [2007 v6 Mustang Automatic, Deluxe ED.] and I have been struggling with this issue for over two years and over several months of research I have come up with nothing. Every forum post that has been started doesn't ever have a real solution other than people saying "get your battery/alt checked" for the thousandth time as well as "just bump up the RPM idle by 100."
It would be fantastic if someone actually had some insight into this, I am tempted to just simply rip out the stock cooling fan and replace it with an aftermarket performance one to see if the original stock fan is requesting too much of a draw.
I cannot say when this issue occurred as it has been happening since the day I bought it used, but I have replaced the battery, alternator, multiple relays, MAF sensor, IAC, and throttle body with all new parts.
Personally, my RPM drops from stock to about 500 or 450 at the lowest, it doesn't always favor a precision point for the RPM drop.
Any help would be much appreciated as I’m getting fed up and tired of wasting money on it
This pretty much sums it up..
EDIT: I think I actually might be onto something, I been reading around and I have seen the topic of fan resistors popping up where if they go bad sometimes the low speed of the fan drops out completely and it goes from the fan fully off to high speed. I did a quick test on this and it seems like this is the issue I am having ( I don't see the fan spinning at all until it kicks into high speed) as well. I will continue to do some more testing on this in the morning...
